1. Field of the Invention
This invention relates to an image sensor for detecting radiation.
2. Description of the Related Art
Image sensors have previously been proposed which comprise a substrate carrying a plurality of switching elements, and a photosensitive element which is associated with each switching element. When appropriate radiation is incident on the photosensitive element, charge is developed which may be stored by the self-capacitance of the photosensitive element, or by an additional charge storage means.
It is desirable to maximise the proportion of the area of the sensor which is occupied by the photosensitive elements, so as to increase the efficiency of the device.
JP-A-04-317373 discloses an image sensor device in which thin film transistors are formed on a substrate, and an insulating film is formed over the transistors. Photoelectric conversion devices are formed over the insulating film, and this enables overlap of the photoelectric conversion devices over the switching transistors. As a result, the proportion of the area of the sensor which is occupied by the photoelectric devices can be maximised.
JP-A-04-317373 is particularly concerned with a mechanically scanning image sensor, in which the minimisation of the size of the sensor is desirable. JP-A-04-317373 is also directed specifically to the production of the insulating film with a flat upper surface, so that the light receiving surface of the image sensor is uniform, and consequently can be maintained at a constant distance from the image to be sensed.
A problem with image sensors of the prior art is that the characteristics of the photoelectric devices degrade over time, notably the dark current in the photoelectric devices.